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authorFederico Motta <federico@willer.it>2018-10-12 11:55:57 +0200
committerJens Axboe <axboe@kernel.dk>2018-10-13 15:40:00 -0600
commit2d29c9f89fcd9bf408fcdaaf515c90a169f22ecd (patch)
treef8dd0b0f7bfa94d4d9251890b38a029de6270b82 /block/bfq-wf2q.c
parenta2fa8a19b75b5a649db2a6bec892ff5e03a23e76 (diff)
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block, bfq: improve asymmetric scenarios detection
bfq defines as asymmetric a scenario where an active entity, say E (representing either a single bfq_queue or a group of other entities), has a higher weight than some other entities. If the entity E does sync I/O in such a scenario, then bfq plugs the dispatch of the I/O of the other entities in the following situation: E is in service but temporarily has no pending I/O request. In fact, without this plugging, all the times that E stops being temporarily idle, it may find the internal queues of the storage device already filled with an out-of-control number of extra requests, from other entities. So E may have to wait for the service of these extra requests, before finally having its own requests served. This may easily break service guarantees, with E getting less than its fair share of the device throughput. Usually, the end result is that E gets the same fraction of the throughput as the other entities, instead of getting more, according to its higher weight. Yet there are two other more subtle cases where E, even if its weight is actually equal to or even lower than the weight of any other active entities, may get less than its fair share of the throughput in case the above I/O plugging is not performed: 1. other entities issue larger requests than E; 2. other entities contain more active child entities than E (or in general tend to have more backlog than E). In the first case, other entities may get more service than E because they get larger requests, than those of E, served during the temporary idle periods of E. In the second case, other entities get more service because, by having many child entities, they have many requests ready for dispatching while E is temporarily idle. This commit addresses this issue by extending the definition of asymmetric scenario: a scenario is asymmetric when - active entities representing bfq_queues have differentiated weights, as in the original definition or (inclusive) - one or more entities representing groups of entities are active. This broader definition makes sure that I/O plugging will be performed in all the above cases, provided that there is at least one active group. Of course, this definition is very coarse, so it will trigger I/O plugging also in cases where it is not needed, such as, e.g., multiple active entities with just one child each, and all with the same I/O-request size. The reason for this coarse definition is just that a finer-grained definition would be rather heavy to compute. On the opposite end, even this new definition does not trigger I/O plugging in all cases where there is no active group, and all bfq_queues have the same weight. So, in these cases some unfairness may occur if there are asymmetries in I/O-request sizes. We made this choice because I/O plugging may lower throughput, and probably a user that has not created any group cares more about throughput than about perfect fairness. At any rate, as for possible applications that may care about service guarantees, bfq already guarantees a high responsiveness and a low latency to soft real-time applications automatically. Signed-off-by: Federico Motta <federico@willer.it> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/bfq-wf2q.c')
-rw-r--r--block/bfq-wf2q.c36
1 files changed, 20 insertions, 16 deletions
diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
index ff7c2d470bb8..476b5a90a5a4 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -788,25 +788,29 @@ __bfq_entity_update_weight_prio(struct bfq_service_tree *old_st,
new_weight = entity->orig_weight *
(bfqq ? bfqq->wr_coeff : 1);
/*
- * If the weight of the entity changes, remove the entity
- * from its old weight counter (if there is a counter
- * associated with the entity), and add it to the counter
- * associated with its new weight.
+ * If the weight of the entity changes, and the entity is a
+ * queue, remove the entity from its old weight counter (if
+ * there is a counter associated with the entity).
*/
if (prev_weight != new_weight) {
- root = bfqq ? &bfqd->queue_weights_tree :
- &bfqd->group_weights_tree;
- __bfq_weights_tree_remove(bfqd, entity, root);
+ if (bfqq) {
+ root = &bfqd->queue_weights_tree;
+ __bfq_weights_tree_remove(bfqd, bfqq, root);
+ } else
+ bfqd->num_active_groups--;
}
entity->weight = new_weight;
/*
- * Add the entity to its weights tree only if it is
- * not associated with a weight-raised queue.
+ * Add the entity, if it is not a weight-raised queue,
+ * to the counter associated with its new weight.
*/
- if (prev_weight != new_weight &&
- (bfqq ? bfqq->wr_coeff == 1 : 1))
- /* If we get here, root has been initialized. */
- bfq_weights_tree_add(bfqd, entity, root);
+ if (prev_weight != new_weight) {
+ if (bfqq && bfqq->wr_coeff == 1) {
+ /* If we get here, root has been initialized. */
+ bfq_weights_tree_add(bfqd, bfqq, root);
+ } else
+ bfqd->num_active_groups++;
+ }
new_st->wsum += entity->weight;
@@ -1012,9 +1016,9 @@ static void __bfq_activate_entity(struct bfq_entity *entity,
if (!bfq_entity_to_bfqq(entity)) { /* bfq_group */
struct bfq_group *bfqg =
container_of(entity, struct bfq_group, entity);
+ struct bfq_data *bfqd = bfqg->bfqd;
- bfq_weights_tree_add(bfqg->bfqd, entity,
- &bfqd->group_weights_tree);
+ bfqd->num_active_groups++;
}
#endif
@@ -1692,7 +1696,7 @@ void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq)
if (!bfqq->dispatched)
if (bfqq->wr_coeff == 1)
- bfq_weights_tree_add(bfqd, &bfqq->entity,
+ bfq_weights_tree_add(bfqd, bfqq,
&bfqd->queue_weights_tree);
if (bfqq->wr_coeff > 1)